’99 Yukon + RS 2 won’t start

My son has a 1999 Yukon (old body style) 4-dr.  It has a Clifford Matrix RS 2 system that had the computer/processor replaced three months ago.  The vehicle will now crank but not start.  When the problem first started, we changed the fuel pump relay.  No difference.  The truck did start the next day.  It then had intermittent problems not starting.  The fuel pump usually makes noise when its "on", and it is not running when the problem occurs.  I understand the fuel pump could be the problem, but I wouldn't think it would have intermittent problems - but would go out for good over a short period of time.  We have pulled the fuse on the RS 2's power supply.  I suspect something with the factory security system.  I don't even know what its functions are (kill power to truck's computer?).  Any insight from a specialist or help from anyone on where to start to get this diagnosed would be greatly appreciated.  Thank you very much.

It probably has passlock.  You need DEI 555L to bypass the factory kill system

 when you turn key on is the theft light flashing if so leave key in run postion and wait till light goes on solid if this is the case check passlock connection on rs unit

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good luck
kirk

Ideally take your truck to a GM dealer. They can quickly pull any codes out of the truck's computer to determine what the problem might be. If your security light on the dash stays on or flashes after starting it is a problem with the Passlock II theft system. It may be your remote starters Passslock interface needs readjusting/replacing. It could also be a problem with the factory equipment. A good mechanic should be able to tell in less than a half hour with the use of a scan tool.

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sparky